Multilevel converters are of interest to use in a number of different power transmission environments. They may for instance be used as voltage source converters in direct current power transmission systems such as high voltage direct current (HVDC) and alternating current power transmission systems, such as flexible alternating current transmission system (FACTS). They may also be used as reactive compensation circuits such as Static VAR compensators.
In order to reduce harmonic distortion in the output of power electronic converters, where the output voltages can assume several discrete levels, so called multilevel converters have been proposed. In particular, converters where a number of cascaded converter cells, each comprising a number of switching units and an energy storage unit in the form of a DC capacitor have been proposed.
Examples of such converters can be found in Marquardt,'New Concept for high voltage-Modular multilevel converter', IEEE 2004, A. Lesnicar, R. Marquardt, “A new modular voltage source inverter topology”, EPE 2003, WO 2010/149200 and WO 2011/124260.
Converter elements or cells in such a converter may for instance be of the half-bridge, full-bridge or clamped double cell type. These may be connected in upper and lower phase arms of a phase leg.
A half-bridge connection in upper and lower arms provides unipolar cell voltage contributions and offers the simplest structure of the chain link converter. This type is described by Marquardt,'New Concept for high voltage-Modular multilevel converter', IEEE 2004 and A. Lesnicar, R. Marquardt, “A new modular voltage source inverter topology”, EPE 2003.
However, there is a problem with the half-bridge topology in that the fault current blocking ability in the case of a DC fault, such as a DC pole-to-pole or a DC pole-to-ground fault, is limited.
One way to address this is through the use of full-bridge cells. This is described in WO 2011/012174. Series connection of full-bridge cells offers four quadrant power flows through the energy storage element of the cell capacitor as well as DC fault voltage blocking capability by imposing a reverse voltage. However, the use of full-bridge cells doubles the number of components compared with a half-bridge cell.
One way to reduce the number of components combined with a retained fault current limiting ability is through mixing the cells of the half- and full-bridge type. Half of the cells may then be full-bridge cells used for imposing the reverse voltage due to the rating of the cascaded converter cells. This is for instance described in WO 2011/042050. The mixing of cells reduces the number of components further while retaining a good fault current limitation ability.
However there is still room for improvement with regard to component reduction combined with fault current limitation.